SYNTHESIS OF IN-SITU COMPOSITES THROUGH SOLID-STATE REACTIONS - THERMODYNAMIC, MASS-BALANCE AND KINETIC CONSIDERATIONS

The applications of thermodynamics, mass balance and kinetics to the s ynthesis of in situ composites through solid-state reactions are discu ssed. The proper choice of starting materials, the principles governin g the diffusion paths and the formation of desirable microstructures a re considered. The use of stability diagrams for rationalizing diffusi on and reactions during composite synthesis is also demonstrated. Appl ying these principles to the synthesis of NbSi2-SiC composites, it is shown that the proper starting materials are NbC1-x and Si. The probab le microstructure is an aggregate type, composed of NbSi2 and SiC. Pre liminary experimental results of the study of bulk NbC1-x-Si diffusion couples annealed at 1300 degrees C for 60 h revealed that the microst ructure is indeed a two-phase mixture of NbSi2 and SiC. Discontinous S iC particles with an average size of 1 mu m are homogeneously disperse d in the NbSi2 matrix. This microstructure is considered favorable for high-temperature structural composite application. Further characteri zation of mechanical and other properties for this material are curren tly being performed.